1. Field of the Invention
The present invention relates to a compressor and, more particularly, to an apparatus and method for controlling an operation of a reciprocating compressor.
2. Description of the Prior Art
In general, a reciprocating compressor (hereinafter, for simplicity, referred to as “reciprocating motor compressor”), which is operated by a linearly reciprocating electric motor without a crank shaft for converting a rotational motion to a linear motion, has less friction loss, and thus, can provide a higher compression efficiency than other compressors.
When the reciprocating motor compressor is used for a refrigerator or an air-conditioner, a compression ratio of the compressor can be varied to control the cooling capacity by varying a stroke voltage applied to the reciprocating motor of the compressor.
The conventional controlling of a reciprocating motor compressor will now be described with reference to FIG. 1.
FIG. 1 is a block diagram showing the construction of an apparatus for controlling an operation of a reciprocating compressor in accordance with a conventional art.
As shown in FIG. 1, the conventional apparatus for controlling an operation of a reciprocating compressor includes a current detector 6 for detecting a current applied to a motor (not shown) of a reciprocating compressor 4; a voltage detector 6 for detecting a voltage applied to the motor; a stroke calculator 7 for calculating a stroke estimate value of the reciprocating compressor based on the detected current value, the detected voltage value and a parameter of the motor; a first comparator 8 for comparing the calculated stroke estimate value and a pre-set stroke reference value, and outputting a first difference value according to the comparison result; a second comparator 1 for comparing a current operation frequency applied to the reciprocating compressor 4 and an operation frequency reference value, and outputting a second difference value according to the comparison result; a stroke controller 2 for outputting a voltage control signal used for varying the voltage applied to the motor of the compressor based on the first difference value outputted from the first comparator 8, and generating an operation frequency control signal used for varying the current operation frequency based on the second difference value outputted from the second comparator 1; and an inverter 3 for varying the current operation frequency according to the operation frequency control signal of the stroke controller 2, and varying the voltage applied to the motor according to the voltage control signal.
Herein, a mechanical resonance frequency of the reciprocating compressor is detected based on the value of the current applied to the reciprocating compressor 4 and the stroke estimate value, and the detected mechanical resonance frequency is determined as the operation frequency reference value.
The operation of the apparatus for controlling the operation of the reciprocating compressor will now be described.
First, the current detector 6 detects a current applied to the motor (not shown) of the reciprocating compressor 4 and outputs the detected current value to the stroke calculator 7. At this time, the voltage detector 5 detects a voltage applied to the motor and outputs the detected voltage value to the stroke calculator 7.
The stroke calculator 7 calculates a stroke estimate value (X) of the reciprocating compressor 4 by substituting detected current value, the detected voltage value and the parameter of the motor to equation (1) shown below, and applies the calculated stroke estimate value (X) to the first comparator 8.
                    X        =                              1            α                    ⁢                      ∫                                          (                                                      V                    M                                    -                  Ri                  -                                      L                    ⁢                                          i                      _                                                                      )                            ⁢                              ⅆ                t                                                                        (        1        )            wherein ‘R’ is a motor resistance value, ‘L’ is a motor inductance value, ‘α’ is a motor constant, VM is the value of the voltage applied to the motor, ‘i’ is the value of the current applied to the motor, and ī is a time variation rate of the current applied to the motor, namely, a differential value (di/dt) of ‘i’.
Thereafter, the first comparator 8 compares the stroke estimate value and the stroke reference value and applies a first difference value according to the comparison result to the stroke controller 2.
Meanwhile, the second comparator 1 compares the current operation frequency and an operation frequency reference value and applies a second difference value according to the comparison result to the stroke controller 2.
The stroke controller 2 outputs a voltage control signal used for varying the voltage applied to the motor of the reciprocating compressor based on the first difference value outputted from the first comparator 8 to the inverter 3, and outputs an operation frequency control signal used for varying the current operation frequency based on the second difference value outputted from the second comparator 1 to the inverter 3.
The inverter 3 varies the current operation frequency according to the operation frequency control signal and varies the voltage applied to the motor according to the voltage control signal.
FIG. 2 is a graph showing a waveform of a current applied to the motor of the reciprocating compressor when the reciprocating compressor is controlled by using the apparatus for controlling the operation of the reciprocating compressor in accordance with the conventional art.
As shown in FIG. 2, when the voltage inputted to the apparatus for controlling the operation of the reciprocating compressor is low or when a load is changed, the waveform of the current applied to the motor of the reciprocating compressor is severely distorted.
For example, the conventional apparatus for controlling the operation of the reciprocating compressor controls the voltage applied to the reciprocating compressor and does not control the waveform of the current passing through the reciprocating compressor. Therefore, when the voltage inputted to the apparatus for controlling the operation of the reciprocating compressor is low or when a load is changed, the waveform of the current applied to the motor of the reciprocating compressor is severely distorted. In addition, since the waveform of the current applied to the motor of the reciprocating compressor is not a sine wave (because the current waveform is distorted), when the mechanical resonance frequency of the reciprocating compressor is detected based on the value of the current applied to the reciprocating compressor and the stroke estimate value, an error is generated and efficiency of the reciprocating compressor is degraded due to a harmonic component of the current applied to the reciprocating compressor.
As mentioned above, the conventional apparatus for controlling the operation of the reciprocating compressor has the following problems.
That is, when the voltage in putted to the apparatus for controlling the operation of the reciprocating compressor is low or when a load is changed, the waveform of the current applied to the motor of the reciprocating compressor is severely distorted. Accordingly, when the mechanical resonance frequency of the reciprocating compressor is detected based on the value of the current applied to the reciprocating compressor and the stroke estimate value, an error is generated and efficiency of the reciprocating compressor is degraded due to a harmonic component of the current applied to the reciprocating compressor.